This invention relates in general to cast light weight vehicle wheels and in particular to a cast wheel having a reinforced portion comprising a metal matrix composite.
Light weight vehicle wheels formed from a light weight metal, such as aluminum, magnesium or titanium, or an alloy thereof, are replacing all-steel wheels on an increasing number of vehicles. Such light weight wheels provide both a reduction in weight from all-steel wheels and an attractive appearance.
Referring now to the drawings, there is illustrated in FIG. 1 a perspective view of a typical cast vehicle wheel, indicated generally at 10, which is formed in accordance with the prior art. The prior art wheel 10 includes an annular wheel rim 11 which is adapted to carry a vehicle tire (not shown). The wheel rim 11 has an axial inner end 11A which is adjacent to the vehicle (not shown) when the wheel 10 is mounted thereon and an axial outer end 11B opposite from the inner end 11A.
The wheel 10 further includes a circular wheel disk 12 formed across the outer end 11B of the wheel rim 11. The wheel disk 12 has an outer surface 12A which is visible when the wheel 10 is mounted upon the vehicle. Accordingly, the outer disk surface 12A is typically machined to a smooth finish and/or decorated. The wheel disk 12 also has an inner surface 12B which is not visible when the wheel 10 is mounted upon the vehicle. The wheel disk 12 includes an outer ring-shaped sidewall 13 which is formed integrally with the outer end 11B of the wheel rim 11. The sidewall 13 is connected by a plurality of radial spokes 14 to a central wheel hub 15. The spokes 14 define a plurality of openings 16 formed through the wheel disk 12. The openings 16 allow a flow of cooling air to the vehicle brakes while reducing the weight of the wheel 10.
The hub 15 includes a recessed center portion 17 having a large central aperture 18 formed therethrough which can receive the end of a vehicle axle (not shown). The recessed center portion 17 includes a plurality of smaller apertures 19 formed therethrough and spaced equally about the central aperture 18. These smaller apertures 19 receive threaded wheel lugs (not shown) for securing the wheel 10 upon a vehicle.
Because light weight metals typically have a lower strength and Young's modulus than steel, the wheel spokes 14 are usually cast with a thick cross section to provide sufficient strength to support the vehicle, as illustrated in FIG. 2. As best seen in FIG. 3, a pair of reinforcing ribs 20 and 21 are formed on the inner surface of each spoke 14 to provide additional strength to the wheel 10. The reinforcing ribs 20 and 21 extend along the spokes 14 from the wheel rim 11 to the wheel hub 15. While two ribs 20 and 21 are illustrated, a single heavy rib (not shown) also can be used. The thick wheel spokes and reinforcing ribs increase the weight of the cast wheels.
Another technique known in the art for strengthening cast metal articles involves including a metal matrix composite in the casting to reinforce the article. U.S. Pat. No. 4,617,979 discloses a method for doing this which consists of first mixing an inorganic fibrous reinforcing material into the molten metal to form a molten mixture which is then introduced into a mold to cast the article. Upon cooling, a metal matrix composite which includes the casting metal and the reinforcing material is formed and extends throughout the entire article.
U.S. Pat. No. 4,920,864 discloses a melt infiltration technique for casting a piston having a portion of the piston crown formed from a metal matrix composite to reduce cracking induced by thermal cycling. According to the patent, a preformed disk of fibrous material having interstices between the individual fibers is positioned in the piston mold. During the casting process, molten metal is forced under pressure into the interstices in the disk to form a metal matrix composite. Similarly, U.S. Pat. No. 4,705,093 discloses a method for casting a fiber reinforced brake caliper, wherein the reinforcing fibers are contained in the bridge portion of the caliper.